Hydrocarbon conversion processes that utilize crystalline zeolite catalysts have become of considerable industrial importance during the last few decades. This is clear from both the large number of patents that were issued in this field, as well as from the number of scientific and trade papers that have been published. The crystalline zeolites are effective for a variety of hydrocarbon conversion processes, some of which are used in the petroleum industry, and others in processing petrochemicals.
Catalytic cracking and/or hydrocracking of petroleum stocks are processes of major importance, and were so regarded even before crystalline zeolite catalysts became known for these processes. Broadly speaking, the principle purpose of cracking and hydrocracking is to reduce the boiling point of the higher boiling fractions of a crude oil. The zeolite employed in this type of conversion process has a pore size sufficiently large to admit all or nearly all of the molecular components normally found in the feed. Such crystalline zeolites are referred to as "large pore size" molecular sieves, and they are generally stated to have a pore size of from about 8 to about 13 angstroms in diameter. Large pore size zeolites are represented by Zeolites X, Y and L. Because the interior regions of the large pore zeolites are accessible to bulky molecules such as highly branched paraffins, and to all but the most bulky substituted aromatics, the "molecular sieve" property of the zeolite plays a very small role in non-selective boiling point reduction by cracking and hydrocracking. See for example U.S. patents 3,140,249, 3,140,251, 3,140,252, 3,140,253, and 3,271,418, all of which are incorporated by reference for background purposes.
Catalytic dewaxing processes, in contrast with cracking processes that use large pore zeolites, require crystalline zeolites of intermediate pore size as catalyst, and critically depend on the molecular sieve properties of the zeolite. Although catalytic cracking with boiling point reduction also takes place in catalytic dewaxing, the pore size of the zeolite permits only linear and singly methyl-branched paraffins (i.e., the waxes) to enter the interior regions of the crystal where they are cracked to lighter hydrocarbon by-products. These byproducts, principally C.sub.1 -C.sub.4 hydrocarbons and naphtha, are readily separated from the remaining, less volatile "dewaxed" oil. In brief, catalytic dewaxing can be considered to be a relatively mild, shape selective cracking process. It is shape selective because the intermediate pore size of the catalyst inherently converts only the long, thin wax molecules to normally liquid or gaseous hydrocarbons. It is mild because the conversion of the gas oil feed to lower boiling range products is small, e.g. usually below about 35 percent and more normally below about 25 percent. It is operative over a wide temperature range but is usually carried out at relatively low temperatures, e.g. start of run temperatures of about 520.degree. F. are usual.
U.S. Pat. No. 3,700,585 discloses and claims the cracking of paraffinic materials from various hydrocarbon feedstocks by contacting such feedstock with a ZSM-5 type zeolite at about 554.degree. F. to 1312.degree. F., at about 0.5 to 200 LHSV (Liquid Hourly Space Velocity) and in some cases with a hydrogen atmosphere. This patent is based upon work on dewaxing gas oils (particularly virgin gas oils) and crudes although its disclosure and claims are applicable to dewaxing any mixture of straight chain and slightly branched chain and other configuration hydrocarbons. The catalyst may have a hydrogenation/dehydrogenation component incorporated therein. Other U.S. patents teaching dewaxing of various petroleum stocks are U.S. Pat. No. Re. 28,398; U.S. Pat. Nos. 3,852,189; 3,891,540; 3,894,933; 3,894,938; 3,894,939; 3,926,782; 3,956,102; 3,968,024; 3,980,550; 4,067,797 and 4,192,734. The foregoing patents are incorporated herein by reference for background purposes.
U.S. Pat. No. 4,446,007 to F.A. Smith describes an improved hydrodewaxing process wherein an intermediate pore size zeolite is used as catalyst, and in which the high hydrogen consumption and low octane of the naphtha characteristic of the line-out period are improved by raising the reactor temperature in a prescribed manner prior to line out. U.S. Pat. No. 4,247,388 to Banta et al. describes treating ZSM-5 type zeolites to adjust their initially high alpha value (such as by steaming) prior to use as dewaxing catalyst. The treatment improves catalyst performance. U.S. Pat. No. 4,251,676 to M.M. Wu describes an improved process for selective cracking of 1,4-disubstituted aromatic compounds wherein the reactor feed is mixed with ammonia or an organic amine to increase the yield of recyclable olefin cracking product. U.S. Pat. No. 3,816,296 to Haas et al. describes selectively producing midbarrel fuels boiling between 300.degree. and 700.degree. F. from higher boiling feeds containing less than 10 ppm nitrogen, by hydrocracking in the presence of added nitrogen compounds corresponding to 5 to 100 ppm nitrogen. U.S. Pat. No. 3,524,807 to C.T. Lewis describes selectively hydrocracking, with increased yield of heavy naphtha, by maintaining the feed nitrogen content within the range of 25-75 ppm.
Hydrocracked oils that are waxy may be catalytically dewaxed to reduce pour point. Such oils typically contain very little nitrogen and have the advantage that they can be dewaxed at somewhat higher space velocity and with longer cycle life than more conventional gas oil feeds. However, such feeds often produce a naphtha of poor octane number, typically a clear research octane in the low eighties, during both the early transient period and even after the dewaxing unit has lined out. In addition, the octane of the naphtha, after line out, is pourpoint sensitive i.e. with increasing dewaxing severity (pour point from 0.degree. to about -30.degree. F.), the naphtha octanes decrease from about 93 to about 86. Compared with the dewaxed fuel oil, the naphtha by-product is a minor product, representing 3.5 to about 5.1 wt% based on charge, but may be higher for waxier feeds. (See Table III below.) The naphtha represents nonetheless a very valuable by-product of a dewaxing plant.
We now find that doping the low-nitrogen content dewaxable feed with a small amount of a high nitrogen content gas oil and dewaxing the resulting blend to the target pour point produces a light naphtha by-product which has a high research octane number, usually at least about 90, and which may be directly blended into the gasoline pool. Additionally, the octane of the naphtha produced in the presence of dopant is no longer pour point sensitive. This uncoupling of pour point and naphtha octane allows the refiner greater freedom in pour point control. As will be illustrated by example herein below, these improvements can be obtained with only a small proportion of dopant, under which conditions little or no decrease in catalyst activity is observed. This is an unexpected result.